Sprint Galaxy S8/S8+ and Note 8 users can't access LTE after Android 9.0 Pie update23
To make things worse, many Samsung Galaxy Note 8 users report they are affected by the same issue since they updated their phones to Android 9.0 Pie. Moreover, this appears to be the same problem that plagued Samsung Galaxy S10 units at Sprint until recently but has been addressed via a software update.
Sprint announced on its community forum that it's investigating these complaints along with Samsung, but in the meantime, the carrier has decided to place the Android 9.0 Pie rollout on hold. The software update will be released once more when the issues have been corrected.
Although affected Galaxy S8/S8+ and Note 8 devices should still have access to 3G and Wi-Fi for data, you are advised to replace your phone if you absolutely need an LTE working model. To do that, you'll have to contact Samsung or visit a Sprint store location for replacement options.
Unfortunately, if your phone is no longer under warranty and you do not have Sprint Complete, you might have to pay a replacement fee to switch to an LTE working device.
Keep in mind that disabling bands on your Samsung smartphone doesn't fix the issue and can have other unforeseen effects on the device, so don't do that and wait for Sprint to release a software update.